The invention relates generally to magnetic data storage systems and, more specifically, to read signal preconditioning circuitry for use in such systems.
Use of magnetic media for mass storage of digital data in a computer system is widespread. Digital data is generally stored on a magnetic medium in the form of magnetic polarity inversions induced into the surface of the medium. If the medium is a magnetic disk, for example, the data is usually arranged in a series of concentric annuluses on the disk""s surface, known as tracks. To read data from one of these tracks, the disk is rotated at a constant speed, and a magnetic transducer is brought near the rotating track to convert the alternating magnetic field emanating from the track surface into an analog electrical signal. One type of magnetic transducer which is widely used for reading digital data from a magnetic medium is a magretoresistive (MR) head.
An MR head is a device whose resistance varies with the applied magnetic field. In this regard, the head is capable of converting magnetic field variations produced by a rotating track into a time varying voltage or current in an electrical circuit. MR heads offer many advantages over other types of magnetic transducers and, consequently, are increasingly being used in magnetic data storage systems. For example, MR heads are more sensitive than other types of read heads, such as thin film heads, and produce a stronger read signal. Also, MR heads have a better frequency response than other types of heads which use inductive coils as a sensing means. In addition, the read signal produced by an MR head is relatively insensitive to the relative velocity between the head and the medium, as is the case with other types of heads, because it is the level of the applied magnetic field which is sensed by an MR head and not the rate of change of magnetic flux lines through a coil. This is an advantage in systems where head/medium velocity may vary over a significant range. Lastly, because MR heads are not capable of writing data on a magnetic medium, magnetic data storage systems which use MR read heads must include a separate head to perform the write function. Using a separate head for reading and writing allows each head to be separately optimized for performing its singular task which can greatly improve the performance of a magnetic data storage system.
As illustrated in the characteristic of FIG. 1, the relationship between the resistance of an MR head and the applied magnetic field is nonlinear. This nonlinear characteristic can produce problems in the conversion of the magnetic field variations emanating from the medium into the time varying electrical signal. For example, the nonlinear nature of the MR head may cause the time varying electrical signal produced by the head to look nothing like the magnetic signal applied to the head. To overcome this problem, a bias current is generally applied to the head to move the quiescent operating point of the head to a more linear region of the resistance characteristic. With reference to FIG. 1, it is seen that maximum linearity in the operation of an MR head is obtained by biasing the head at point A, i.e., the most linear point on the characteristic. It may be desirable, however, to bias the head at another point, such as point B or point C, to maximize a conversion parameter which may be more important than linearity, such as signal to noise ratio (SNR). As a consequence of such biasing, the output signal of the head may be asymmetrical about a zero volt baseline, such as output waveform 10 in FIG. 1 corresponding to bias point B. In addition to biasing effects, other factors may also exist which result in an asymmetrical read signal, such as off-track effects.
Because of the high data densities being stored on magnetic media today, read signals are comprised of relatively narrow electrical pulses and read signal asymmetry can make detection of the stored data bits difficult. For example, in a disk drive using a peak detector, the difference in the magnitude of the positive and negative peaks of the read signal complicates, among other things, the setting of the threshold levels used in detecting the peaks. Alternatively, in a disk drive using a partial response maximum likelihood (PRML) channel, the difference in the magnitudes of the positive and negative peaks of the read signal complicates the sampling of the signal which must be performed before maximum-likelihood detection can occur. A need therefore exists for an apparatus which is capable of overcoming the problems created by an asymmetrical read signal produced by an MR head.
In addition to the above-described asymmetry, MR heads are also known to produce a shift in the baseline of the signal read from the magnetic medium. FIG. 2 illustrates a read signal having such a baseline shift 12. It has been proposed that this baseline shift 12 is caused by the presence of parasitic magnetic dipoles along the edges of the data track which are sensed by the MR head during readback. An expanded discussion of this phenomena can be found in xe2x80x9cTrack Edge Phenomena in Thin Film Longitudinal Media,xe2x80x9d IEEE Transactions on Magnetics, Vol. 25, No. 5, September 1989, by Su et al. In addition, shifted baselines may also be present in read signals produced by other types of heads, such as thin film heads. As with asymmetrical peaks, the presence of shifted baselines can complicate the detection of the data stored on the magnetic medium. A need therefore exists for an apparatus which is capable of overcoming the problems created by the baseline shift of the read signal.
The present invention relates to an apparatus for preconditioning a read signal produced by a magnetoresistive (MR) head, before performing detection, to reduce the problems associated with pulse asymmetry and baseline shift. In one embodiment, the invention is capable of correcting the amplitude asymmetry of a read signal before the signal is detected. In another embodiment, the invention is capable of removing baseline shift from a read signal before the signal is detected. In this regard, the invention is capable of increasing the reliability, i.e., decreasing the read error rate, of magnetic data storage systems using conventional detection methods.
In one aspect of the present invention, a magnetic data storage system is provided which includes signal preconditioning circuitry for reducing read signal distortions, namely baseline shift and positive and negative pulse amplitude asymmetry. More specifically, this aspect of the invention includes: a magnetic medium, a magnetic head for sensing data stored on said medium and for creating an analog read signal, detection circuitry for converting the analog read signal into a digital signal representative of the data stored on the magnetic medium, and read signal preconditioning circuitry located between the head and the detection circuitry for reducing distortions in the read signal before the read signal reaches the detection circuitry.
The magnetic medium may include any magnetic material capable of storing data, such as magnetic tape, floppy disks, and hard disks. The magnetic head may include, for example, a thin film head, a ferrite head, or, most preferably, a magnetoresistive head. The detection circuitry can include any circuitry capable of converting an analog read signal into a digital signal representative of data stored on the magnetic medium, such as PRML circuitry, peak detection circuitry, or decision feedback channel circuitry.
In one embodiment, the preconditioning circuitry comprises a baseline shift reduction circuit for reducing the baseline shift created by the head during reading. In another embodiment, the preconditioning circuitry comprises pulse asymmetry reduction circuitry for reducing the asymmetry of the read signal, around a zero-volt baseline, created by the head. In yet another embodiment, the preconditioning circuitry may include both baseline shift reduction circuitry and pulse asymmetry reduction circuitry.
In another aspect of the present invention, circuitry is provided for equalizing the amplitudes of the positive and negative pulses of a read signal created by an MR head. More specifically, this aspect of the invention includes: a magnetic medium; a magnetoresistive (MR) head for reading data from the medium and for creating a read signal wherein the amplitude, with respect to a zero voltage baseline, of the positive and negative peaks of the read signal are unequal; and circuitry for equalizing the amplitudes of the positive and negative peaks of the read signal. The circuitry for equalizing the amplitudes of the positive and negative peaks may include, for example, a transconductance multiplier such as the well known Gilbert cell multiplier. In another embodiment, the circuitry for equalizing may include circuitry for separating the positive peaks of the read signal from the negative peaks of the read signal and/or circuitry for separately adjusting the amplitudes of the positive and negative peaks. The circuitry for equalizing may also include circuitry for recombining the positive and negative peaks after their amplitudes have been separately adjusted.
In a third aspect of the present invention, circuitry is provided for reducing the baseline shift of the read signal. More specifically, this aspect of the invention includes: a magnetic medium; a magnetic head for reading data from the medium and for creating a read signal with a undesired baseline shift; and circuitry for reducing the baseline shift of the read signal. The magnetic head may include, for example, a thin film head, a ferrite head, or, most preferably, a magnetoresistive head. The circuitry for reducing the baseline shift may include circuitry for providing low frequency boost and/or phase shift to the read signal. In another embodiment, the circuitry for reducing the baseline shift may include frequency discrimination circuitry such as, for example, reactive filtration circuitry. This embodiment may also include amplification circuitry for amplifying the read signal after it has passed through the frequency discrimination circuitry.